It is generally the case at semiconductor manufacturing plants that an IC package comprising a plastic encapsulated semiconductor integrated circuit chip (which will hereafter be referred to as an IC chip) is subjected to an electrical property test and a reliability test called a burn-in test prior to its shipment in order to separate good from bad product. The electrical property test is carried out for the purpose of inspecting the input-output characteristics, pulse characteristics, noise leeway, and the like of the IC chips. In the burn-in test, the IC packages which pass the electrical property test are placed in an oven and operated for a certain period of time under an electric voltage source which is larger than the rated value by approximately 20 percent at a high temperature, 120 degrees centigrade, for example. The IC packages which perform unsatisfactorily in the burn-in test are discarded as being unsatisfactory and only the IC packages which continue to perform normally are shipped out as satisfactory products.
The IC package is generally subjected to the burn-in test loaded in a socket. Such sockets are installed on a printed wiring substrate and, with a large number of contacts provided in the socket engaging the leads of the IC in a 1:1 relationship, the IC package is electrically connected to a test device through the printed wiring substrate. Accordingly, the IC package must be accurately positioned inside the socket so that each contact element will accurately engage the corresponding lead.
FIGS. 11 and 12 show typical prior art structure for positioning an IC package in a socket. According to the structure shown in FIG. 11, an IC package 100 is loaded in the socket on base 102 and either the side or the corner part of package 100a is positioned by a rib 102a provided on base 102. A large number of contacts 104 are arranged around base 102 at a pitch corresponding to the leads 100b of IC package 100 and contact elements 104 are elastically displaced as shown by the dashed line 104' by the force coming from a compressive driving means that is not shown in the drawings, thereby making contact under added force with the tip part of leads 100b.
With regard to FIG. 12, guide post 106 having a tapered surface 106a is disposed at a prescribed location on the base 102 so that when the IC package 100 is placed on base 102 leads 100b at the end of each row of leads are guided and positioned by tapered surface 106a.
If there is an error in the relative positional relationship between package 100a and leads 100b in the IC package 100, according to the positioning method as described relative to FIG. 11, there is a danger of leads 100b being misaligned with contacts 104 even if the package 100a is properly positioned on the base 102.
Also, with regard to FIG. 12, if the lead 100b at the end of a lead row hits the tapered surface 106a of guide post 106 while being held by a carrier, it can be bent as shown by the dashed line 100b', with a result that there is a possibility not only for this lead but also the lead row as a whole to be moved out of position on the base 102. That is, when an IC package 100 is loaded in the burn-in socket, typically a carrier arm of an automatic unit or the like carries the IC package 100 over to the top of base 102. If the loading position of the carrier arm deviates even a slight amount in this connection, lead 100b at the end severely impacts the tapered surface 106a of the guide post 106 thereby bending the lead.
As the pitch between the leads of the IC package become narrower and as the leads become thinner along with improvements of the integration density of the IC chip, the shortcomings of the conventional positioning methods as described above become all the more pronounced.